Matsu'ura Mansaku is seventeen and is one of the seven protagonists in RAINBOW. He is nicknamed Cabbage. He has been detained for violence and underage drinking. He performed an act of violence while drunk, leaving the victim with grave injuries.

Despite his huge figure, Mansaku is quite gentle in nature and probably the most calm amongst the seven cell mates. He showed his caring nature by wiping away the blood clots from Sakuragi's body after he was heavily beaten and left senseless for being in a fight with new cell mates by the inspector in charge.